This invention pertains to power conversion apparatus, and in particular, to apparatus which enables the conversion of DC power or single-phase AC power to three-phase AC power. More specifically, it pertains to such a system wherein the AC frequency supplied to a three-phase load is independent of the frequency available from the DC or single-phase AC source, and is selectively variable.
There are many circumstances where it is required, or at least desired, to operate a three-phase load, such as a three-phase motor, in a facility where only, for example, conventional, 120-volt, single-phase power is available, or, where a DC power supply is the only power source. Further, there are many applications where, in addition, it is desired to supply a three-phase load with a selectively variable-frequency voltage (independent of the source frequency) for the purpose (in the case of a motor load) of enabling variable-speed control.
Conversion circuitry which has been proposed in the past to accomplish the task of driving such a load from such a source is typically quite complex and expensive, in that it usually employs a large number of relatively expensive power switches (switching transistors) to accomplish the required conversion. In most cases, six such switches are employed.
An important object of the present invention is to provide apparatus which is capable of performing the kind of conversion just mentioned and which employs a minimum number (no more than four) of such switches.
A related object is to provide such apparatus which is distinguished by relatively simple circuitry and relatively low cost.
Yet another object of the invention is to provide apparatus of the type generally outlined which is capable of supplying a voltage output having a variable frequency--completely independent of the frequency of the source voltage.
The surprising capability of the apparatus of the invention to meet these objectives results from the recognition that one of the input terminals in a selected three-phase load can be directly grounded, with each of the two other output terminals supplied, under computer control, a sinusoidal voltage artifact each having a common selectively variable frequency which is totally independent of the source frequency. The circuitry employed in supplying each of these two voltage artifacts requires only two power switches for each artifact.
Various other objects and advantages which are attained by the invention will become more fully apparent as the description which now follows is read in conjunction with the accompanying drawings.